In a conventional construction machine such as a bulldozer, the "independent two pump system" is employed to ensure hydraulic power required for carrying out operation of the attached implement such as a blade and ripper and turning of the vehicle body at the same time. In this system, two separate pumps are used, one being an implement hydraulic pump for supplying hydraulic power to the implement, and the other being a steering hydraulic pump for supplying hydraulic power to the steering hydraulic motor.
In this known system, variable displacement hydraulic pumps are used as the implement hydraulic pump and the steering hydraulic pump, for the purpose of preventing possible engine stalling when these pumps are simultaneously operated. By use of such pumps, horse power consumption can be reduced and engine torque rise and the amount of torque required for travelling in the low speed range are set high.
A prior art relating to the present invention is disclosed in Japanese Patent Publication (KOKAI) No. 57-133940 (1982). This publication discloses a construction machine which employs a single motor for operating three hydraulic pumps (two variable displacement pumps and one fixed displacement pump). When this construction machine is not in turning motion, the sum of horse power input to the variable displacement pumps which are not for use in turning operation is set to a value obtained by subtracting a certain amount from the total output horse power of the motor, whereas the discharged fluid of the fixed displacement pump used for turning operation is allowed to join the fluid pressure circuit which is not for turning operation. On the other hand, when the construction machine is in turning motion, the sum of horse power input to the variable displacement pumps not for use in turning operation is first made to be substantially equal to the total output horse power of the motor and is then allowed to decrease as the discharged horse power of the fixed displacement pump for turning operation increases. In this way, the total of horse power input to the three hydraulic pumps is maintained to be substantially constant.
Systems using variable displacement pumps both for the implement hydraulic pump and for the steering hydraulic pump, like the conventional independent two pump system, require an auxiliary machine such as a turbo-charger or after-cooler in order to increase engine torque rise, resulting in poor cost performance and poor fuel economy. Two variable displacement hydraulic pumps complex in structure are essential for the conventional two pump system, and installation of two variable displacement pumps makes the entire system bulky and costly.
The related art disclosed in the publication No. 57-133940 is designed to allow the discharged fluid of the pump for turning operation to join the fluid pressure circuit not for use in turning operation and is therefore inapplicable to construction machines such as described in the invention in which there is a big difference in pressure between the implement hydraulic circuit and the steering hydraulic circuit.
The present invention is directed to overcoming the above problems and the primary object of the invention is therefore to provide a control method and system for a construction machine, which employ the independent two pump system including an implement hydraulic pump and a steering hydraulic pump, the method and system capable of preventing engine stalling with inexpensive system structure and without increasing fuel cost.